Therefore I added <LangVersion>14</LangVersion> to the vbproj file.
But that generates a different error when building from Visual Build:
vbc : Command line error BC2014: the value '14' is invalid for option 'langversion'

When I set <LangVersion>11.0</LangVersion>, Visual Studio also shows
error BC36637: The '?' character cannot be used here
=> the <LangVersion> property works correctly.